1. Field of the Invention
The present invention relates to a method and device for inspecting front and back surfaces of a magnetic disk for defects thereon. The invention more particularly relates to a method and device for inspecting front and back surfaces of a magnetic disk for defects thereon, which are suitable to optically inspect the front and back surfaces of the magnetic disk simultaneously and thereby to detect a dent and a scratch on the front and back surfaces of substrates of the magnetic disk and detect foreign substances attached to the front and back surfaces of the magnetic disk.
2. Description of the Related Art
A device for optically inspecting a magnetic disk for microscopic defects on surfaces of a substrate thereof is needed to have more enhanced inspection sensitivity and higher throughput for inspection. Schemes of achieving higher throughput for inspection are described in JP-A-H3-186739 and JP-A-H5-21561. Specifically a laser light source is used to increase the intensity of illumination light and a high-sensitivity sensor detects light reflected on and scattered from a substrate. However, if the intensity of the light emitted is too high, the surface of the substrate may be damaged. Thus, it is necessary to irradiate the substrate with light having a controlled intensity and detect, with the maximum sensitivity, reflected and scattered light from a defect that is present on the surface of the substrate by irradiating the substrate with light having a limited intensity. JP-A-H5-21561 describes a structure that includes a photomultiplier as a structure that allows reflected and scattered light to be detected with the maximum sensitivity.
JP-A-H6-118015 describes an inspection system for an increase in the throughput. This inspection system includes a substrate reversing mechanism located between a front surface inspecting section that inspects the front surface of a magnetic disk and a back surface inspecting section that inspects the back surface of the magnetic disk. The inspection system sequentially inspects the front and back surfaces of the magnetic disk, thereby improving the throughput for inspection.
In addition, JP-A-H8-22619 describes an inspection system that uses a handling device to transfer a magnetic disk between multiple types of magnetic disk inspection devices and performs a series of inspections on the magnetic disk. In the inspection system, one of inspecting sections, which optically inspects a foreign substance, optically inspects front and back surfaces of the magnetic disk simultaneously.
In addition, JP-A-2009-175121 describes a structure for irradiating both surfaces of a disk with light simultaneously for defect inspection.
Scattered light from a microscopic defect has characteristics such that the smaller the size of a defect is, the less the amount (per unit area) of light scattered from the defect, and the scattered light spreads over the whole space above the defect. Thus, in order to detect the microscopic defect with high sensitivity by means of light having the same intensity, it is necessary to efficiently collect the light scattered from the defect in a larger region and thereby to detect the defect.
In order to collect the scattered light in the larger region, the size of an objective lens that is used to collect light scattered from a substrate is increased and the size of the numerical aperture of the objective lens is increased. It is, however, necessary to prevent the objective lens from interfering with other parts of an inspection device when the objective lens is installed in the inspection device. Thus, practically, increasing the size of the numerical aperture by increasing the size of the objective lens has a limit.
Generally, an inspection device has a mechanism for maintaining and driving the substrate on the side of the back surface of the substrate. Thus, the inspection device has a narrow space for an optical system on the side of the back surface of the substrate.
It is, therefore, difficult to provide the same optical system (that is used for inspection) on the side of the back surface of the substrate as that provided on the side of the front surface of the substrate, when inspections on both surfaces of a substrate are simultaneously to be performed.
In addition, when the size of the numerical aperture is increased by increasing the size of the objective lens, it is necessary to increase the size of a focusing lens that is used to focus light (scattered light) collected by the objective lens onto a detection surface of a detector. Since the lens group is increased in size, a lens barrel that holds the lens group is also increased in size. Thus, the optical system for inspection is also increased in size. It is, therefore, difficult to reduce the size and weight of the inspection device.
JP-A-H3-186739 and JP-A-H5-21561 do not consider that, for improvement in inspection sensitivity for a defect, the size of the numerical aperture of an objective lens is increased without increasing the size of the inspection device for increasing the amount of scattered light (to be detected) relative to the amount of emitted light.
In the method (described in JP-A-H6-118015) for inspecting the surfaces of a magnetic disk for defects thereon, the magnetic disks are sequentially transferred between the two inspecting sections so that the defects that are present on the front and back surfaces of the magnetic disk are inspected and the throughput for inspection of both surfaces of the magnetic disk is improved.
However, since the two inspecting sections are arranged side by side and the reversing mechanism is provided between the two inspecting sections, an area on which to install the inspection system will be large.
In addition, JP-A-H8-22619 describes that the front and back surfaces of the magnetic disk are optically inspected simultaneously. However, a specific structure for optically inspecting the surfaces simultaneously is not disclosed.
In addition, JP-A-H4-62457 describes that a structure including light sources and a camera is provided on each side of the front and back surfaces of a disk and optically inspects both surfaces of the disk simultaneously for defects thereon. The light sources are provided on both sides of the disk in bilateral symmetry with respect to the central axis of the disk and emit light on one surface of the disk in an oblique direction. However, JP-A-H4-62457 does not describe a structure that uses an optical system having a large numerical aperture on both sides of the disk to detect light scattered from microscopic defects.